1. Field of the Invention
The present invention relates to an optical film used in a liquid-crystal display device (LCD) or the like. More specifically, it relates to an optical film adapted for improvement in display characteristic and luminance, and a liquid-crystal display device using the optical film.
2. Description of the Related Art
An LCD is used in a personal computer or the like. There has been a rapidly increasing demand for the LCD in recent years. Measures to make all constituent members of the LCD thin to thereby make the LCD thin as a whole have been examined. One of the constituent members which has been improved most remarkably in terms of reduction in thickness is a backlight unit. The backlight unit used in a liquid-crystal panel is constituted by a laminate of various films such as a laminate of a light pipe, a diffusing plate and a condensing sheet. Accordingly, interference or sticking caused by lamination of films looks like a defect when an image is displayed. Particularly the necessity for making the constituent members of the backlight unit (such as a light pipe, a diffusing plate and a condensing sheet) thin, reducing clearances between the members or removing some of the members for the sake of simplification has occurred with the strong requirement of reduction in thickness of the liquid-crystal panel in recent years. Hence, there a rises a problem that a defect or a sticking portion due to interference fringes (Newton's rings) or sticking caused by lamination of films is made visible when an image is displayed. On the other hand, both reduction in thickness of the backlight unit and improvement in definition of display lead to reduction in luminance of the panel, that is, go against the recent tendency to increase in luminance of the LCD panel. Therefore, a luminance enhancement film becomes essential to supply required panel luminance.
Although measures to insert a diffusing plate between the backlight unit and the liquid-crystal panel or to provide a diffusing layer in the luminance enhancement film or the polarizer itself have been heretofore taken to make the aforementioned defect invisible on the display, the measurements could not satisfy the requirement of reduction in thickness, retention of necessary characteristic and reduction in cost.